Have you tried melting down large amounts of plastic and molding it into large, structurally sound pieces? Seems like it would be a lot easier to just toss them all into a big net bag, or tie them together by the handles. If I just want a recycled plastic boat, well, my milk jugs do get picked up for recycling and I could buy a boat made of recycled plastic. The idea of using them as-is, is about saving time and effort and money.

Actually, I've made armor for Live Action Role Playing groups, from HDPE, culled from discarded sheets and blue barrels used for detergents for cleaning apples. This was done between 1989 to 1992. So could large amounts be melted down and made structurally sound? Absolutely. The only time I experienced structural failures was when I used rivets rather than welding the plastic, when piecemealing it together.

You could throw a bunch of milk jugs into a net or tie them together, to make a raft. Sure! Refugees do it all the time. That isn't even close to what the author did either. I thought he was quite clever for using the materials at hand, but still able to make something that resembled a boat. It's like an art piece you can use.

In regards to your remark about why bother to recycle it down, and make a boat from recycled materials, when you can just buy one. Ok, first off, you are speaking out of ignorance. I have yet to see any companies in the US, and a tiny thimble's amount worldwide, that make boats solely from recycled HDPE materials. Also, on an industrial scale, it'd be expensive to produce one this way, to say the least. So that is the only point you get in your favor. For a clever guy at home, to make one for himself, it's not really as expensive as you seem to think.

A cheap blender bought at a second hand store, an old cookie sheet, his oven, and C clamps are the only hardware you need. If you're already a craftsman, you don't need to spend a dime on C clamps. A second hand blender is 5 to 8 bucks tops at a thrift store. Cookie sheets are like a buck to 3 bucks tops, if you have that. The molds can be made from recovered sheets of plywood, and if you don't have baking parchment or aluminum foil in your cupboard, you made need to buy those. Not really expensive, not even for someone living on minimum wage.

You can even make your own welding sticks with the plastic. And to weld it, if you have a heat gun, you can train your eye and hand coordination to weld the sheets together, or weld the HDPE formed ribs to the sheeting. So a 9mm thick sheet, measuring 30cm x 30cm is around 20 to 24 cups of ground up HDPE milk jugs (depending on how fine you ground them in the blender). Then all it takes is a good design, and some experience in fabrication to piece it all together, into an amazing watercraft. If you so choose to build one.

Milk jugs can be fairly easy to find. One example, go to apartment complexes and post a sign, saying you want to them for the purpose of your project. You'd be surprised how many people will happily give away their empties to you, if it means they can see their trash turned into a real world item, and defy belief it's real. But it is.

I should also add, that you should heat the plastic in a ventilated area, due to HDPE being an irritant. Of course if it's on fire, it's dangerous. From the oven, well unless you're standing right over it and decide to inhale, sure that'll cause you grief. According the OSHA MSDS, HDPE is also a safe plastic for storage and contact with food and water.

So I'm going to agree to disagree with your sentiments towards my terse suggestion, Dream. All the best to your endeavors. :)